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Supporting Information
Silicon Carbide Passive Heating Elements in
Microwave-Assisted Organic Synthesis
Jennifer M. Kremsner and C. Oliver Kappe*
Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, A-8010 Graz, Austria.
Table of Contents
General Experimental Details and Microwave Equipment S2
Heating Profiles for Solvents, Solvent Mixtures, and Reactions S3-S17
Images of Heating Elements S12-S13
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General Experimental Details: 1H NMR and 13C NMR spectra were recorded on a 360 MHz
instrument at 360 and at 90 MHz respectively. Chemical shifts (δ) are expressed in ppm downfield from
TMS as internal standard. The letters s, d, t, q and m are used to indicate singlet, doublet, triplet,
quadruplet and multiplet. FTIR spectra were recorded using KBr pellets. Low resolution mass spectra
were obtained on a LC/MS instrument using atmospheric pressure chemical ionization (APCI) in
positive or negative mode. Analytical HPLC analysis was carried out on a C18 reversed-phase (RP)
analytical column (119 × 3 mm, particle size 5 mm) or a reversed-phase column (150 × 4.6 mm, particle
size 5 mm) at 25 °C using a mobile phase A (water/acetonitrile 90:10 (v/v) + 0.1 % TFA) and B (MeCN
+ 0.1 % TFA) at a flow rate of 0.5-1.0 mL/min. The following gradient was applied: linear increase
from solution 30% B to 100 % B in 7 min, hold at 100% solution B for 2 min. Dry-flash
chromatography was performed on silica gel 60 H (< 45 nm particle size). Melting points were obtained
on a standard melting point apparatus in open capillary tubes. TLC analyses were performed on pre-
coated (silica gel 60 HF254) plates. All anhydrous solvents (stored over molecular sieves), and chemicals
were obtained from standard commercial vendors and were used without any further purification.
Microwave Irradiation Experiments: Heating curves of solvents were recorded using a single-mode
Discover System from CEM Corporation using either custom-made high purity quartz or standard Pyrex
vessels (capacity 10 mL) as appropriate, sealed with a Teflon septum cap. The temperature profiles of
the solvents (power control) were monitored either using a calibrated infrared temperature control
mounted underneath the reaction vessel, or a fiber-optic probe inserted into the reaction vessel protected
by a sapphire immersion well. Chemical transformations in the presence of heating elements (SiC, 10 x
18 mm cylinder) described in this article were either run in a CEM Discover, or in Biotage Emrys
Synthesizer or Initiator Eight EXP instruments in the standard configuration (temperature control,
remote IR temperature sensor, sealed Pyrex vessels). Microwave reactions in multimode instruments
were performed in a SYNTHOS 3000 instrument from Anton Paar GmbH using 100 mL Teflon sealed
reaction vessels and a 16 position rotor. Graphical representations of SiC heating elements (mp ca. 2700
°C, density 3.10 g/cm3, specific heat capacity 650 J/kgK, thermal conductivity (100 °C) 120 W/mK,
thermal expansion (20-1000 °C) 4.1 x 10-8 K-1) and reaction vessels are shown in Figure S10.
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0 50 100 150 200 250 300 350 400Time [s]
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PyrexQuartz
Figure S1. Microwave heating profiles for CCl4 in Pyrex and quartz reaction vessels at constant 150 W
magnetron output power for 5 min (CEM Discover). Single mode irradiation, 4 mL sample volume,
fiber-optic temperature measurement, sealed 10 mL reaction vessel, magnetic stirring. The heating of
the microwave transparent solvent to 109 °C in the case of the Pyrex vessel is the result of indirect
heating by conduction and convection phenomena via the hot surface of the self-absorbing Pyrex glass.
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fiber opticIR
Figure S2. Microwave heating profiles for a standard 10 mL empty Pyrex vessel at constant 150 W
magnetron output power for 10 min (CEM Discover) using fiber-optic or IR temperature measurement.
The self-heating of the conventional Pyrex reaction vessels is clearly demonstrated by comparison of
heating profiles using IR and fiber-optic temperature measurements. Since the IR sensor directly
monitors the surface temperature of the glass (rather than of its contents), the observed effects are more
pronounced using this type of monitoring method. In the experiment described herein, microwave
transparent air was used as medium.
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PyrexQuartz
Figure S3. Microwave heating profiles for ethanol in quartz and Pyrex reaction vessels at constant 150
W magnetron output power (CEM Discover). Single mode irradiation, 4 mL sample volume, IR
temperature measurement, sealed 10 mL reaction vessel, magnetic stirring. There is virtually no
difference between the heating profiles using the microwave transparent quartz and the to some extend
absorbing Pyrex vessel, since ethanol is a strongly microwave absorbing solvent.
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0% EtOH0.9% EtOH2.1% EtOH4.5% EtOH9.0% EtOH16% EtOHpure EtOH
Figure S4. Microwave heating profiles for samples of 2 mL (3.20 g) of CCl4 doped with varying
amounts of ethanol in a 10 mL quartz reaction vessel at constant 150 W magnetron output power. Single
mode microwave irradiation, IR temperature sensor, magnetic stirring (CEM Discover). Profiles were
recorded for: pure CCl4, 30 mg (0.9 %, w/w) ethanol, 70 mg (2.1 %, w/w) ethanol, 150 mg (4.5 %, w/w)
ethanol, 310 mg (9 %, w/w) ethanol, 610 mg (16 %, w/w) ethanol, and pure ethanol . In the case of the
16 % ethanol/CCl4 mixture, the experiment was terminated after ca 150 s at ca 140 °C when the
pressure limit of the instrument (20 bar) was reached (bp CCl4 = 76 °C). Even small amounts of
strongly microwave absorbing ethanol are capable of changing the overall absorption characteristics of
the solvent mixture to a large extent allowing significant heating by microwave dielectric effects.
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0.0 M0.001875 M0.00375 M0.0075 M0.015 M0.03 M0.06 M0.09 M
NaCl Concentration:
Figure S5. Microwave heating profiles at 150 W constant magnetron output power for aqueous NaCl
solutions of varying salt concentrations. Single mode irradiation, 5 mL sample volume, sealed 10 mL
quartz reaction vessel, IR temperature measurement, magnetic stirring (CEM Discover). Pure water is
rather difficult to heat by microwave irradiation, in particular at higher temperatures as the loss tangent
(tan δ = 0.123 at 25 °C) decreases significantly with increasing temperature.
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0.0 M0.03 M0.015 M0.0075 M
TBAB Concentration:
Figure S6. Microwave heating profiles for aqueous tetrabutylammonium bromide (TBAB) solutions at
constant 150 W magnetron output power. Single mode irradiation, 5 mL sample volume, sealed 10 mL
quartz reaction vessel, IR temperature measurement, magnetic stirring (CEM Discover).
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2 W 5 W 25 W 50 W 75 W 100 W
Figure S7. Microwave heating profiles for the ionic liquid N-butyl-N’-methylimidazolium
hexafluorophosphate (bmimPF6). Single mode microwave irradiation, 5 ml sample volume, sealed 10
ml quartz reaction vessel, fiber-optic temperature probe, magnetic stirring (CEM Discover). Profiles
were recorded at 2 W, 5 W, 25 W, 50 W, 75 W and 100 W magnetron output power. The very strong
coupling characteristics are demonstrated by the fact that only 5 W of microwave energy suffices to heat
the ionic liquid to 160 °C within less than 5 minutes. For experiments reaching temperatures above 200
°C the cooling profiles using compressed air cooling are also shown.
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Ionic Liquidhexane
Figure S8. Differential heating of a biphasic mixture of N-butyl-N’-methylimidazolium
hexafluorophosphate (bmimPF6, 2.5 mL, bottom layer IL) and hexane (2.5 mL, top layer, hex) as
monitored by fiber-optic probe temperature measurements at 2 W constant power. Single mode
microwave irradiation, 4 mL total sample volume, sealed 10 mL quartz reaction vessel, fiber-optic
temperature probe measurements at different positions in the vessel (see insert), no stirring (CEM
Discover). The heating curves are strongly dependent on the position of the temperature probe,
reflecting the different temperatures in the two immiscible phases (differential heating). There is a more
than 40 °C temperature gradient after 100 s between the strongly absorbing ionic liquid phase and the
poorly absorbing hexane layer. Temperature measurements using the standard remote IR sensor
technique, either from the bottom (CEM Discover platform) or from the side (Biotage Emrys and
Initiator platforms) will lead to erratic results and will make it difficult to reproduce experiments using
different equipment.
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Time [s]
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without PHECarboflonWeflon
Figure S9. Microwave heating profiles (150 W constant magnetron output power) for CCl4 in the
presence of the fluoropolymer-derived heating elements WeflonTM (5 x 20 mm cylinder, ca. 0.8 g,
source: Milestone) and Carboflon® (6 x 13 mm cylinder, ca. 0.9 g, source: CEM). Single mode
microwave irradiation, 4 mL sample volume, sealed 10 mL quartz reaction vessel, IR temperature
sensor, magnetic stirring (CEM Discover).
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B C D
E F
Figure S10 (see next page for caption)
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Figure S10. Compatibility of SiC passive heating elements (PHEs) with common reaction vessels
used in microwave synthesis. (a) PHE cylinders: 10 x 18 mm (4.35 g) and 10 x 8 mm (1.94 g). (b) PHE
cylinder (10 x 18 mm) inside a standard 10 mL microwave vial used in CEM Discover or Biotage
Emrys/Initiator platforms, solvent volume 2 mL. Note that the design allows for magnetic stirring of the
reaction mixture during the reaction with a standard stir bar placed below the PHE cylinder. (c) PHE
cylinder (10 x 8 mm) inside a standard 5 mL conical microwave vial used in Biotage Emrys/Initiator
platforms, solvent volume 2 mL. (d) PHE cylinder (10 x 18 mm) inside a standard 20 mL microwave
vial used in Biotage Initiator EXP platforms, solvent volume 15 mL. (e) PHE inside a 100 mL Teflon
microwave vessel for use in an Anton Paar SYNTHOS 3000 multimode reactor. In the presence of a 2
cm magnetic stir bar, efficient stirring is possible. For clarity the vessel wall has been cut. (f) IR thermal
image of a 10 x 8 mm SiC heating element exposed to microwave irradiation inside a multimode
microwave cavity. The recorded temperature after 2 min of irradiation at 400 W was 247 °C (shown) or
262 °C after 1 min at 600 W power.
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CCl4hexanetoluenedioxaneTHF
Figure S11. Microwave heating profiles for non-polar solvents (2 mL) in the presence of a SiC heating
element. Single mode microwave irradiation, 150 W constant magnetron output power, sealed 10 mL
Pyrex reaction vessel, IR temperature sensor, magnetic stirring (CEM Discover). The following solvents
are displayed: CCl4, toluene, THF, dioxane and hexane. The maximum reached temperatures reflect a
pressure of ca 20 bar at which the experiments were aborted and cooling by compressed air commenced.
In the absence of the heating elements there is very little heating for most solvents (see Table 1 in the
manuscript).
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[°C] dioxane
chloroformtoluenexyleneMeCNwaterCCl4
Figure S12. Microwave heating profiles for 100 mL samples of non-polar solvents (10 mL solvent,
SiC heating element, and 2 cm stir bar per vessel; 10 vessels in a 16-vessel rotor). Multimode
microwave irradiation, 1000 W constant power for 10 min, sealed 100 mL Teflon reaction vessel, gas
balloon temperature sensor, magnetic stirring (Anton Paar SYNTHOS 3000). The following solvents
are displayed: H2O, toluene, CCl4, CHCl3, dioxane, xylene and acetonitrile.
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Pow
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T
IR
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Figure S13. Heating profiles for the Diels-Alder cycloaddition of 2,3-dimethylbutadiene and
acrylonitrile in toluene, carried out on a 100 mmol scale (total solvent volume ca 120 mL). The
experiment was carried out using a multimode microwave instrument (Anton Paar SYNTHOS 3000) in
Teflon reaction vessels housed in a 16 vessel rotor. Heating ramp to 240 °C (7 min), temperature control
using the feedback from the reference vessel temperature measurement (constant 240 °C, 7-27 min), and
forced air cooling (27-50 min). The reaction was performed in 4 Teflon vessels each containing ca 30
mL of reaction volume (see Experimental Section for details). Shown is the temperature measurement in
one reference vessel via internal gas balloon thermometer (T), the surface temperature monitoring of the
4 individual vessel by IR thermography (IR 1-4), and the magnetron power (P, 0-1400 W). Note that the
power of the magnetron suffices to follow a heating ramp from 20 to 240 °C in 7 min. After the
maximum temperature has been reached ca 320 W power is used to keep the reaction temperature at 240
°C. Also note that the individual IR vessel surface temperatures deviate by less than 10 °C. For clarity,
the pressure graph is not shown.
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T QuartzT Pyrex + SiCT PyrexP QuartzP Pyrex + SiCP Pyrex
Figure S14. Microwave heating profiles for acetonitrile (2 mL) in temperature control mode. Single
mode microwave irradiation, 120 °C set maximum temperature, sealed 10 ml Pyrex or quartz reaction
vessels, IR temperature sensor, magnetic stirring (CEM Discover). Displayed are temperature and
power profiles for: a) heating in a quartz vessel without SiC element; b) heating in a Pyrex vessel
without SiC element; c) heating in a Pyrex vessel with SiC element. It is clearly demonstrated that the
heating of acetonitrile at 120 °C in a quartz reaction vessel requires the highest power (200 W). Due to
the self-absorbance of the Pyrex vessel the required energy is significantly reduced (140 W) when the
experiment is conducted in this vessel type. In the presence of the SiC heating element less than 50 W
of energy are required to maintain a solvent temperature of 120 °C.